What this change does is increase the size of the coast to convert ocean to coast up to COAST_SIZE squares from the ocean.

I have already made a fork of Totestra that does just this. Look for BigCoast.py and BigCoast.diff at Totestra.html.

[1] The only way I play Civ4 right now is with the Legends of Ancient Arabia mod, which renames the Galley a "Lateen". It has something to do with the fact I am 1/8 Arabian.

How do I add more rivers to a map?

It is possible to edit the amount of rivers by editing one of the numeric parameters. If you're comfortable using a text editor, you can edit the following line in Totestra.py:

self.RiverThreshold = 4

Try making this 3 or 2.

How do I adjust the amount of humidity (rain fall) on a map?

This is a parameter that can be edited by altering the source code; look for this line:

self.minimumRainCost = 0.01

And make 0.01 a higher value.

How do I make continents fatter or thinner?

Increasing the "Continent amount" will cause the map generator to make thinner and more stringy continents.

How do I add mod-specific terrain to a map

Changing the terrain Totestra/PerfectWorld generates to make mod-specific terrain is a little tricky. To get an idea of how it is done, look at the changes I made to it to make more arid/desert maps for the Legends of Ancient Arabia mod, which should give you an idea where to start changing the code to add support for mod-specific terrain: